The present invention relates generally to methods of forming granules by coating a grain-like core with a powder material utilizing a liquid binder and, more particularly, to such methods wherein diamond, synthetic diamond or cubic boron nitride grains are coated with a metal powder for use in press-molding operations such as in the manufacture of metal-bonded diamond tools.
As is known, many high technology cutting and abrading tools are conventionally fabricated of a suitable metal with minute grains of diamond, synthetic diamond or cubic boron nitride generally uniformly embedded in the metal within the cutting or abrading components of the tools. Basically, such tool components are formed by conventional powder metalurgical techniques wherein the abrasive grain material is initially mixed with a powdered metal or combination of metals, after which the mixture is utilized in a conventional press-molding operation to bond and shape the mixture into the desired tool component. As desired, sintering heat may be applied as part of the pressing operation or, alternatively, the press-molded component may be subjected to a subsequent sintering procedure. Within the relevant industry, tools fabricated in this manner are commonly referred to as metal bonded diamond tools.
Conventionally, the composition of such metal bonded diamond tools is identified according to four parameters: (a) the type of abrasive grain material utilized, (b) the grain size of the abrasive material expressed in terms of the range of standardized U.S. screen mesh sizes through which the grains will pass, (c) the concentration of the abrasive grains as a proportion of the total volume of the grain/metal conglomerate with a 100 concentration designating a total grain weight of 4.4 carats per cubic centimeter of conglomerate, and (d) the grade of the abrasive grain/metal conglomerate representing the relative hardness thereof which derives from the grain retaining strength of the metal-to-grain bond. Whereas the abrasive type, grit size and concentration are measurable objective parameters which thereby have become standardized within the industry, the grade of metal-bonded diamond tools is a more subjective designation which depends in large part on the manufacturing know-how and skill of the tool maker. As will be understood, however, one of the most critical factors in determining the grade of a metal-bonded diamond tool is the degree of homogenous distribution of the abrasive grains within the grain/metal conglomerate, which directly affects the performance and life of the tool.
According to one conventional process of manufacturing metal bonded diamond tools, pre-weighed quantities of the abrasive grain and powdered metal raw materials in appropriate proportions to provide the desired concentration are initially mixed in batch form in a suitable mixing apparatus, following which the batch mixture is manually fed in individual weighed charges into each die cavity of the pressing apparatus for performance of the press-molding operation. As will be apparent, this process suffers the disadvantages of being highly labor intensive and, further, being extremely dependent on the particular skill of the technician performing the process, which often results in inconsistent quality and grade. Additionally, the process of feeding the mixture into the die cavities cannot be automated due to the poor flowability of metal powder and the tendency of the abrasive grains to segregate from the metal powder during any automated feeding operation.
In an alternative conventional method, the metal powder is initially processed with an agglomerating binder solution in a suitable granulating apparatus to convert the metal powder into a granular form having improved flowability. The granules and the abrasive grains are then weighed and mixed in predetermined proportions as in the first-described process, preparatory to the press-molding operation. According to this method, the mixture of the metal granules and abrasive grains may be fed automatically into the die cavities of the pressing apparatus, as a result of the enhanced flowability provided by the granular form of the metal powder. However, the metal granules typically vary considerably more in size than the abrasive grains and, in any event, differ in density from the abrasive grains. Disadvantageously, these differences in physical properties tend to produce segregation of the metal granules and the abrasive grains during mixing, particularly in any automated feeding operation, and, further, result in an uneven distribution of the abrasive grains in the tool component ultimately produced, which deleteriously affects the grade of the tool component.
In attempting to solve the problems of the foregoing methods, it has been proposed to preliminarily form the abrasive grain and metal powder raw materials into composite granules by using an agglomerating binder to coat the abrasive grains with the metal powder. According to one such method in conventional use, a pre-weighed batch of abrasive grains is charged into a suitable granulating apparatus, such as a conventional tumbling-type granulator, with pre-measured quantities of the metal powder and binder then being fed alternately or simultaneously through separate charge ports into the granulator during its tumbling operation to progressively build a metal powder coating on the abrasive grains. The desired grain concentration is achieved either by continuing the granulating process until all metal powder is coated onto the abrasive grains or by sieving the metal-coated granules to a predetermined granule diameter range calculated theoretically on the assumption that each granule contains a single abrasive grain. In practice, however, it has been found that only a relatively small percentage of the granules produced in fact contain only a single abrasive grain, many of the granules either having no abrasive grain or containing multiple grains. This disadvantageous result occurs because the binder tends to cause the metal powder not only to adhere to the abrasive grains but also to agglomerate to itself while, at the same time, the granulating apparatus does not achieve optimal dispersion of the abrasive grains within the granulating chamber so that at least some of the grains tend to segregate and therefore several grains may be bound into a single granule. As a net ultimate result, the abrasive grains are thereby segregated and unevenly distributed within the tool component produced from the granules, which of course negatively affects the grade of the tool component. Furthermore, the interior surfaces and component parts of the granulating apparatus which come into contact with the abrasive grains during the granulating process are subjected to severe wear, which significantly limits their useful life and thereby increases the cost of the granulating operation.
It is also known in the relevant industry to apply a coating of single or multiple layers of thin metal film on abrasive grains by eletro-plating, chemical plating or vacuum deposition of the metal film on the abrasive grains. Representative examples of such processes are disclosed in South African patent application Nos. 70/3466, filed May 22, 1970, and 70/3653, filed May 29, 1970, each in the name of DeBeers Industrial Diamond Division, Ltd. In such processes, the grain retaining strength of the metal-to-grain bond is improved by taking advantage of the chemical reaction between the diamond grain surface and the metal layer achieved at a sintering temperature.